IR Absorption Induced by Hydrogenation

The bonding of H atoms in amorphous Si has received considerable attention, with review articles summarizing the experimental results and their interpretation in terms of local bonding groups by Cardona (1983) and by Lucovsky and Pollard (1984). The result that is pertinent here 

The first explanation has been discussed in more detail by Lucovsky in Pankove et al. (1985) but can be restated simply as follows the Si—-H force constant is reduced by the slight attraction of the nearby B atom, as shown in Fig. 15b. Hence the frequency of the Si—H stretching vibration is slightly reduced. 

The second explanation, based on Cardona s (1983) discussion of the local field effect, requires that the dielectric cavity have about the size of a monovacancy. 

Using Raman scattering, Steigmeier of RCA Zurich has found in one of our samples a weak structure at 1875 cm-1. This finding, which implies a lack of inversion symmetry, is consistent with our Si—H model. 

Further important support for our model comes from the work of Johnson (1985), who measured the absorption peaks of hydrogenated and 

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Maier and coworkers have shown that it is possible to induce by irridation using the appropriate absorption band, a 1,2-hydrogen shift in silenes and in silylenes29,158. Thus, it is possible to switch photochemically between silylenes and silenes. Michl, West and coworkers have used this approach to isomerize dimethylsilylene 285 and 1-methylsilene 26 several times (equation 70). Due to the clean formation of 285 from the diazido precursor 286153 it was possible to measure the IR transition moment directions for both 26 and 285156. 

The thermal vibrations of atoms in molecules lead to absorption bands in the infrared (IR) region (Bellamy, 1964 Colthup et al., 1964 Hesse et al., 1984). IR bands are most intense if a dipole is induced by the vibration (OH, NH, CH, C=0, C=N). The mass of the interacting atoms Ml and M2 and the bond strengths defined by a force constant f determine the wave number n or energy of an infrared absorption band v = K(f/M ) , where the reduced mass is M = MjM2/Mj+M2 and K is a constant conversion factor. The frequency n for the CH-stretch vibration is around 2900 cm for C=0 close to 1700 cm. Hydrogen bonds lead to a broadening and low-frequency shift of OH and NH vibrations (3400 cm —> 3200 cm ). 

The accuracy of determining the hydrogen content from IR absorption spectra can be undermined in several ways. For example, light-induced degradation of photoconductivity of weakly doped films and alloys of amorphous hydrogenated silicon, known as the Staebler-Wronski effect [135], is clearly observed upon illumination by radiation with energy /iv > 1.17 eV over several hundreds of hours [150]. Usually, the formation of dangling bonds when weak Si—Si bonds are broken as a result of the nomadiative recombination of an electron-hole pair in the tails of the conductivity and valence bands is responsible for the effect [133, 151]. 

Finally, reference is made to the dynamic dichroic absorption difference, which is related to an absorption change induced by an external time-dependent perturbation. Dynamic absorption spectroscopy is performed with linearly polarized IR light and serves for example to detect intermolecular hydrogen bonds in polyamides of the structure... 

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